Method and apparatus for storing and distributing encryption keys

ABSTRACT

A plurality of infrastructure system devices other than a mobile station is divided into a plurality of pools. An intrakey is utilized to encrypt messages passed between infrastructure system devices in the same pool, and an interkey is utilized to encrypt messages passed between infrastructure system devices of different pools.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of prior application Ser. No.09/785,849, filed Feb. 16, 2001.

FIELD OF THE INVENTION

This invention relates to encrypted communications, including but notlimited to air interface communication within secure communicationsystems.

BACKGROUND OF THE INVENTION

Encrypted voice and data systems are well known. Many of these systemsprovide secure communication between two or more users by sharing onepiece of information between the users, which permits only those usersknowing it to properly decrypt the message. This piece of information isknown as the encryption key variable, or key for short. Loading this keyinto the actual encryption device in the secure communication unit is abasic requirement that allows secure communication to occur. To retainsecurity over a long period of time, the keys are changed periodically,typically weekly or monthly.

Encryption is known to be performed on an end-to-end basis within acommunication system, i.e., encrypting a message at the originatingcommunication unit (also known as a mobile station), passing ittransparently (i.e., without decryption) through any number of channelsand/or pieces of infrastructure to the end user's communication unit,which decrypts the message.

The Terrestrial Trunked Radio (TETRA) communication standard ispresently utilized in Europe (hereinafter TETRA Standard), withpotential for expansion elsewhere. The TETRA Standard calls for airinterface, also known as air traffic or over-the-air, encryption. Airinterface encryption protects information on the air interface betweenthe infrastructure and the mobile subscriber. The TETRA standard callsfor an authentication center, also known as a key management facility orkey management center, to generate, distribute, and authenticateencryption keys and users. The TETRA standard does not, however, specifyhow to implement an authentication center, nor how to generate,distribute, and authenticate key material to system devices or mobilestations for information traversing through the infrastructure or SwMI(Switching and Management Infrastructure), as it is referred to in theTETRA Standard.

The TETRA standard fails to provide definition to minimize burden tocall processing and bandwidth, provide encryption and authentication ina manner tolerant to equipment faults, support wide-area communications,and to store keys for all communication units without undue storageburden at local sites.

Accordingly, there is a need for a method and apparatus for providing asecure infrastructure for a communication system that utilizes airinterface encryption and generates, distributes, and authenticatesencryption keys and users without causing undue burden to callprocessing, bandwidth, security, and storage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a secure communication system in accordancewith the invention.

FIG. 2 is a block diagram showing key distribution pools in accordancewith the invention.

FIG. 3 and FIG. 4 are block diagrams showing key storage within acommunication system in accordance with the invention.

FIG. 5 is a diagram showing key storage and authentication informationdistribution within a communication system in accordance with theinvention.

FIG. 6 is a diagram showing authentication information storage andauthentication decision making within a communication system inaccordance with the invention.

FIG. 7 is a diagram showing authentication of a mobile station by anauthentication center in accordance with the TETRA Standard.

FIG. 8 is a diagram showing authentication of an authentication centerby a mobile station in accordance with the TETRA Standard.

FIG. 9 is a diagram showing key storage and authentication informationdistribution between a communication system and a mobile station inaccordance with the invention.

FIG. 10 is a diagram showing a key pull within a communication system inaccordance with the invention.

FIG. 11 is a diagram showing a key push within a communication system inaccordance with the invention.

FIG. 12 is a diagram showing distribution of a static cipher key to abase station within a communication system in accordance with theinvention.

FIG. 13 is a diagram showing distribution of a static cipher key to amobile station within a communication system in accordance with theinvention.

FIG. 14 is a diagram showing distribution of a common cipher key to amobile station and a base station within a communication system inaccordance with the invention.

FIG. 15 is a diagram showing distribution of a group cipher key to abase station within a communication system in accordance with theinvention.

FIG. 16 is a diagram showing distribution of a group cipher key to amobile station within a communication system in accordance with theinvention.

FIG. 17 is a flowchart showing a method of key persistence at a site ina communication system in accordance with the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

The following describes an apparatus for and method of providing asecure infrastructure for a communication system that utilizes airinterface encryption and generates, distributes, and authenticatesencryption keys and users without causing undue burden to callprocessing, bandwidth, security, and storage. System devices are dividedinto groups or pools and encryption keys are defined to provide securetransfer of key material among the system devices.

A block diagram of a secure communication system that is comprised of aplurality of zones is shown in FIG. 1. The secure communication systemis comprised of a plurality of system devices that comprise theinfrastructure of the system. A Key Management Facility (KMF) 101transfers security data, such as session authentication information andencryption keys, to a User Configuration Server (UCS) 103, that forwardsthe information and data to the appropriate zone based on configurationdata within the UCS 103. Communications for a first zone are provided bya plurality of system devices including a Zone Manager (ZM) 105, a ZoneController 107 that includes a Home Location Register (HLR) 109 and aVisited (also known as a Visitor or Visitors') Location Register (VLR)11 1, an air traffic router (ATR) 113, and a plurality of base stations(BSs) 115 and 117 located at a plurality of communication sites withinthe first zone. Communications for a second zone are provided by aplurality of system devices including a ZM 119, a ZC 121 that includesan HLR 123 and a VLR 125, an ATR 127, and a plurality of BSs 129 and 131located at a plurality of communication sites within the second zone.The BSs 1 15, 117, 129, and 131 communicate with a plurality of mobilestations (see FIG. 4). The ZCs 107 and 121 communicate via a network133, such as a local area network or a wide area network such as an IP(internet protocol) network. Only two zones and their associated systemdevices are shown for the sake of simplicity, although any number ofzones may be successfully incorporated in the secure communicationsystem.

For the sake of simplicity, not all system devices will be shown in eachFigure, but rather a representative set of system devices thatillustrates a particular concept will be provided. Similarly, not allkey material is shown stored in each system device for the sake ofspace. Each message containing a key, key material, configuration, orother information is transferred with an related identity (ID) such asITSI or GTSI, although the ID is generally not shown in the drawings forspace considerations.

The KMF 101 is a secure entity that stores the authentication key (K)for each mobile station (MS) or communication unit, such as a portableor mobile two-way radio, Direct Mode Operation (DMO) gateway, receiver,scanner, or transmitter (for example, see devices 401, 403, and 405 inFIG. 4). The KMF 101 provides a random seed (RS) and associated sessionauthentication keys (KS and KS′) for each mobile station associated withthe secure communication system. The KMF 101 also imports/generatesvarious air interface keys, such as Static Cipher Key (SCK), GroupCipher Key (GCK), and Common Cipher Key (CCK), for distribution in thesystem. The KMF 101 functions as the authentication center (AuC), asreferred to in the TETRA communication standard, in the system.Typically, there is one KMF server per system, although there may be oneor more KMF clients per system.

The UCS 103 is a single point of entry for configuration data in thesystem. In the preferred embodiment, the UCS 103 stores and distributessession authentication information, such as RS, KS, and KS′, to theappropriate home zone in the system. The UCS 103 functions as a non-realtime distribution point for session authentication information in thesystem.

The ZM 105 or 119 is a management database for a zone. In the preferredembodiment, the ZM 105 or 119 stores session authentication information,such as RS, KS, and KS′, for the zone managed by the particular ZM 105or 119. The ZM functions as a non-real time storage facility forauthentication information in the zone.

The ZC 107 or 121 performs real time authentication for the mobilestations in its zone. The ZC uses the session authenticationinformation, such as RS, KS, and KS′, to perform the real-timeauthentication. The HLR 109 or 123 stores session authenticationinformation for each MS that has the HLR 109 or 123 as its home. The VLR111 or 125 stores session authentication information for each MSvisiting the VLR's 111 or 125 zone. The ZC 107 or 121 performs real-timedistribution of its home mobile stations' session authenticationinformation when the MS roams outside its home zone. In the preferredembodiment, an HLR 109 or 123 and VLR 111 or 125 are part of each zonecontroller and perform on behalf of the same zone for which the zonecontroller is associated. The HLR 109 or 123 and VLR 111 or 125 may bepart of other system devices or may be stand alone devices. The derivedcipher key (DCK) is generated during authentication. The ZC 107 or 121generates and distributes the DCK for the MS to the BSs 115, 117, 129,and 131 that require the DCK for secure communications.

The ATR 113 or 127 is the conduit used by the KMF 101 to send rekeymessages or key updates to an MS, such as SCK and GCK. The KMF 101 sendskey updates for mobile stations to the home zone ATR 113 or 127 fordissemination. All rekey acknowledgments (ACKs), whether infrastructureor MS originated, pass through the ATR 113 or 127 to the KMF 101.

Each BS 115, 117, 129, and 131 receives and transmits authenticationmessages over the air interface. Each BS 115, 117, 129, and 131 acts asa transmitter for its associated ZC 107 or 121 and as a receiver for theMS in the system. The BS 115, 117, 129, or 131 uses DCK for airinterface encryption with the MS. The BSs 115, 117, 129, and 131 areresponsible for sending key material to the MSs 401, 403, 405, and 407.The result of some of these operations (SCK, GCK) is sent back to theKMF 101. Because each base site is comprised substantially of one ormore base stations, the terms base site (or site) and base station areused interchangeably herein, both sharing the acronym BS. In thepreferred embodiment, a TETRA site Controller (TSC) connects all thebase stations at a site, stores key material, and distributes keymaterial to the base stations as needed, thereby making keys availableto all base stations at a site. Thus, when a key is said to be stored ata base station or a base site, in the preferred embodiment, the TSCactually provides storage for the base station for key material. Becausekey storage and distribution and other key-related functions may beperformed by a base site, base station, or TSC, these terms areconsidered interchangeable for the purposes of this document.

The Mobile Station (MS) authenticates the system and/or is authenticatedby the system using a challenge-response protocol. Each MS has its ownkey, K, for use during authentication. Each MS is assigned to one HLR,which typically remains the same. Each MS is also associated with onlyone VLR in the zone in which the MS is presently located. An MS is notregistered on a system until the MS is active and has passedauthentication.

FIG. 2 is a block diagram showing key distribution pools. Using a singlekey encryption key (KEK) to encrypt keys for distribution system wide isa convenient choice, although a single KEK would result in degradedsecurity due to the higher likelihood that the KEK would be compromisedand the resultant compromise would affect the whole system. Using adifferent KEK for each system device would be more secure, but wouldburden storage within system devices and add unnecessary delays to callprocessing. FIG. 2 shows a system for using KEKs that is more securethan a single system-wide key, yet not as burdensome as a different KEKfor each system device. Two types of KEKs are assigned to confidentiallydistribute key material (such as air interface keys, sessionauthentication information, data utilized to generate encryption keys,and other key-related material) to the system devices of theinfrastructure of a system: intrakeys and interkeys. KEKs are 80 bits inthe preferred embodiment.

The first type of KEK is an intrakey, also referred to as an intrapoolkey or intra-zone key, KEK_(Z). The system devices are divided intopools or groups 201, 203, 205, and 207. Each pool is assigned its ownunique intrakey, KEK_(Z). In the preferred embodiment, each pool ofdevices corresponds to a zone in the communication system, and each poolhas a mutually exclusive collection of system devices, i.e., each systemdevice only belongs to one pool. The first pool 201 utilizes KEK_(Z1) toencrypt key material, such as encryption keys and/or sessionauthentication information, for transfer within the first pool (or zonein the preferred embodiment) and comprises the first zone controller ZC1107 and its associated BSs 115, 117, and 211. The second pool 203utilizes KEK_(Z2) to encrypt key material for transfer within the secondpool (or zone in the preferred embodiment) and comprises the second zonecontroller ZC2 121 and its associated BSs 129, 131, and 213. The thirdpool 205 utilizes KEK_(Z3) to encrypt key material for transfer withinthe third pool (or zone in the preferred embodiment) and comprises thethird zone controller ZC3 223 and its associated BSs 225, 227, and 229.The fourth pool 207 utilizes KEK_(Z4) to encrypt key material fortransfer within the fourth pool (or zone in the preferred embodiment)and comprises the fourth zone controller ZC4 215 and its associated BSs217, 219, and 221. In the preferred embodiment, the intrakey is used bya zone controller to distribute key material to base sites/base stationswithin its zone. KEK_(Z) is also used by the KMF 101 to distribute SCK.

The second type of KEK is an interkey, KEK_(M), also referred to as aninterpool key or inter-zone key. The interkey is used to encrypt keymaterial sent between pools or zones in the preferred embodiment, orwithin a certain group 209 of system devices, particularly from the KMF101. In the preferred embodiment, the interkey is used by the KMF 101 todistribute GCK and individual authentication information to theinfrastructure. In the preferred embodiment, the interkey is stored inone system device in each zone, in each zone controller 107 and 121, andis also stored in the KMF 101. The connections shown between the KMF 101and the zone controllers 107, 121, 215, and 223 are virtual connectionsin the preferred embodiment, in that other devices, such as the UCS 103and ZMs 105 and 119, are physically located between the KMF 101 and zonecontrollers 107, 121, 215, and 223. The UCS 103 and ZMs 105 and 119 passencrypted key information in a transparent manner between the KMF 101and zone controllers 107, 121, 215, and 223, i.e., the UCS 103 and ZMs105 and 119 do not decrypt or encrypt the information, thus no storageof a KEK is required at the UCS 103 and ZMs 105 and 119, although keymaterial may be stored in encrypted form at the UCS 103 and ZMs 105 and119.

Preferably, a message is encrypted by one of an intrakey and aninterkey, typically using TA31 (decrypted using TA32), based on a systemdevice to which the message is forwarded. For example, when the messageis intended for a system device in a zone other than the zone containingthe sending device, the interkey is used. When the message is intendedfor a system device in the same zone as the zone containing the sendingdevice, the intrakey is used. In the preferred embodiment, when the KMF101 encrypts key material, such as SCK, CCK, SAI, and GCK, with eitherthe interkey or intrakey, the KMF 101 uses TA31.

For example, from time to time, key material is distributed from the HLRto a VLR and then to the base sites within the zone of the VLR. In thiscase, the key material is encrypted by KEK_(M) and passed transparentlyfrom HLR to VLR. The target VLR decrypts the key material using itsKEK_(M) and re-encrypts it with the KEK_(Z) of the zone for distributionto sites within the zone.

Each system device that contains an infrastructure KEK has its ownunique infrastructure or protection key, KI, in the preferredembodiment. The protection key is only utilized to decrypt/encrypt KEKssent by the KMF 101 to the infrastructure system devices. Preferably,the KI is only able to be loaded by a key variable loader and is notable to be updated with an OTAR (over-the-air rekey) operation. Inaddition to distribution by the KMF 101, the KEKs may also be manuallyprovided with a Key Variable Loader. KI is 128 bits long in thepreferred embodiment.

As shown in Table 1 below, KEK_(M) is only stored by the zonecontrollers 107 and 121 and the KMF 101. The intrakey KEK_(Z) is heldonly by the KMF 101, base stations/sites, and zone controller 107 and121 within each zone. Each zone has a unique KEK_(Z). Each system devicehas its own KI. TABLE 1 Distribution of Key Encryption Key TypesInfrastructure Element Zone 1 Zone 2 Zone Controller (HLR, VLR) KI₁,KEK_(M), KEK_(Z1) KI₂, KEK_(M), KEK_(Z2) Base Sites KI₃, KEK_(Z1) KI₄,KEK_(Z2)

The use of intrakeys and interkeys strikes a unique tradeoff betweensecurity and key management complexity as well as speed of callprocessing. The KMF 101 need only maintain one interkey plus oneintrakey for each pool or zone in the system. If a KEK_(Z) iscompromised, the affect and response is localized to that zone, ratherthan the whole system, and KI remains intact to redistribute a newKEK_(Z) to that zone. KEK_(M) is stored only at the KMF 101 and the HLR109 and 123 and VLR 111 and 125 in each zone, which devices aretypically more physically protected from an attack. If KEK_(M) iscompromised, the KMF 101 changes KEK_(M) in the ZCs 107 and 121, leavingthe sites unaffected.

Five basic types of air interface keys are used to encrypt air interfacetraffic in the secure communication system: a Static Cipher Key (SCK), aCommon Cipher Key (CCK), a Group Cipher Key (GCK), a Derived Cipher Key(DCK), and a Modified Group Cipher Key (MGCK). Three basic types of keysare used between the system devices: an Infrastructure Key (KI) alsoknown as a protection key, an inter-zone or inter-pool key encryptionkey also known as an interkey (KEK_(M)), and an intra-zone or intra-poolkey encryption key also known as an intrakey (KEK_(Z)).

The Static Cipher Key (SCK) is the most basic of the air interface keysand is used to encrypt inbound (MS to infrastructure) and outbound(infrastructure to MS) information when authentication and/or dynamicair interface encryption is not available. Thus, the generation anddistribution of this key has no relation to authentication.

The Derived Cipher Key (DCK) is a session key derived within theauthentication procedure. The DCK changes each time an authentication isperformed with the MS and the infrastructure, also called the SwMI inthe TETRA Standard. The DCK is used for inbound traffic encryption. TheDCK is also used for outbound individually addressed traffic to the MS.DCK is used when using dynamic air interface encryption operating inTETRA Standard security class 3.

This Common Cipher Key (CCK) is a group key in the sense that multipleMSs have the same CCK. Unlike the GCK, however, the CCK has no relationto a particular talkgroup (TG). The CCK is geographically specific,i.e., the CCK serves all units within a given location area. Thelocation area as defined in the TETRA standard may be as small as a siteor a big as an entire system. Each unit within a location area uses thesame CCK. Group communications in the outbound direction use CCK whenthere is no GCK/MGCK available for that group call. CCK is used for theencryption of outbound group traffic and identities only. Inboundidentities are encrypted with CCK when DCK is in use.

Indirectly, the Group Cipher Key (GCK) is used to encrypt outboundtalkgroup calls. In the preferred embodiment, a GCK is defined for eachtalkgroup in the system. Actually, the GCK is only indirectly used forthe encryption of traffic information; the modified group cipher key(MGCK), which is a derivative of the GCK, is directly used for trafficencryption. GCK is never used for the actual encryption of traffic as itis considered a long term key.

The Modified Group Cipher Key (MGCK) is used to encrypt outboundtalkgroup call traffic. MGCK is formed by the combination of GCK andCCK. Each GCK has a corresponding MGCK defined in for a location area.

Each infrastructure element has an infrastructure or protection key, KI,that is used as the encryption key for any infrastructure key encryptionkey updates. KI is similar in function to the authentication key, K, ina mobile station. In the preferred embodiment, KI is updated only by aprovisioning device such as a key variable loader. In the preferredembodiment, infrastructure key encryption key (KEK) updates cannot beperformed without this key.

Each zone controller has an interkey, KEK_(M), also referred to as aninter-zone or inter-pool key, which is used to encrypt all key trafficpassed between the KMF and each zone. KEK_(M) is also used by the zonecontroller to pass GCK, CCK, and DCK, as well as session authenticationinformation, between zones. In the preferred embodiment, one KEK_(M) ispresent in the KMF and each of the zone controllers in each system.

Each zone has its own intrakey, KEK_(Z), also referred to as anintra-zone or intra-pool key. The intrakey is used to encrypt all keytraffic within the zone, between the zone controller and each of thesites within the zones. Each base site and zone controller has the sameKEK_(Z) in a zone. The KMF stores the KEK_(Z) for each zone in thesystem.

A method of the present invention establishes an expected lifetime, orrekey interval, for an encryption key. Table 2 below shows example rekeyintervals for each key stored in the secure communication system. Whenthe expected lifetime for an encryption key expires, i.e., when therekey interval occurs, the encryption key is replaced.

A number of storage locations for each type of system device within acommunication system is determined. For example, one KMF 101, one UCS103, one ZM 105 or 119 per zone, one zone controller 107 or 121 perzone, one HLR 109 or 123 per zone, one VLR 111 or 125 per zone, and anumber of sites and corresponding base stations per site depending onthe coverage requirements for each zone. Based on the expected lifetimefor each encryption key and the number of storage locations for eachsystem device, a type of system device is assigned to store eachencryption key, and the encryption keys are stored at the system deviceof the assigned type. For example, derived cipher keys are stored atbase stations and in the HLR/VLR, common cipher keys are stored at basestations, modified group cipher keys are stored at base stations, andgroup cipher keys that are stored at HLRs and VLRs.

Table 2 shows the target (user) of each key and the rekey interval,i.e., time between changes or updates of the specific key in a preferredembodiment. For example, the MGCK, which is a combination of CCK andGCK, is updated whenever CCK is changes and whenever GCK is changed.Table 2 may be changed by the KMF operator. TABLE 2 KEY TARGET REKEYINTERVAL SCK All MS, all BS 1 year/or if compromised DCK MS, BS, HLR,VLR <24 hrs, whenever unit authenticates CCK group (TG HLR), all MS, 24hrs all BS GCK group (TG HLR) 6 months MGCK group(BS, MS) 24 hrs -Minimum of CCK, GCK interval KI All devices using KEK_(Z) Never changesor KEK_(M) (BS, ZC KEK_(Z) zone 6 months/or if compromised KEK_(M)system 6 months/or if compromised

PC (personal computer) based software programs exist that provision bothmobile stations and infrastructure system devices with keys. A moresecure method utilizes the capabilities of the Key Variable Loader(KVL), or key loader for short, to load keys into the infrastructuredevices as well as the MS. The key loader has a hardware basedencryption device for the securing of keys stored within the device. TheKVL may obtain keys directly from the KMF acting as a store and forwardagent in order to disseminate the key encryption keys to the variousdevices.

Although a KVL is a very secure way to provide keys, it is a very timeconsuming process to use one or more KVLs to provide keys at each systemdevice and mobile station. A method of key management is needed to storeand distribute the KEKs and other key material to system devices such aszone controllers and base sites.

The KMF 101 is responsible for the generation, key distribution, andtracking of most of the air interface keys (not DCK or MGCK) in thesystem. The base sites 115 and 117 and each zone controller 107 serve asa proxy to the KMF 101 for key distribution. The KMF 101 distributes keymaterial to the zones through the UCS 103, ZMs 105 and 119, and/or ATRs113 and 127 depending on the key being distributed. The KMF 101processes acknowledgement information from the ATR 113 and 127 tomaintain currency of the system devices and MSs 401, 403, 405, and 407.FIG. 3 and FIG. 4 show key material storage within the communicationsystem.

As shown in FIG. 3, the KMF 101 stores a protection key and associatedKEK(s) for each system device. The KMF 101 stores a protection(infrastructure) key, an interkey, and an intrakey for each zonecontroller. For example, the first zone controller 107 is associatedwith the keys KI_(ZC1), KEK_(M), and KEK_(Z1). The KMF 101 stores thesekeys encrypted by a hardware key and the first zone controller 107stores KI_(ZC1) and the encrypted KEK_(M) and KEK_(Z1). The KMF 101stores a protection key and intrakey, both protected by a hardware key,for each BS. For example, the KMF 101 and the first BS 115 both storethe protection key KI_(BS1) and the intrakey KEK_(Z1). In the preferredembodiment, the KMF 101 stores keys encrypted/protected by a hardwarekey.

Prior to distribution of a KEK in the preferred embodiment, the KMF 101encrypts KEKs with the protection key, KI, and the use of encryptionalgorithms TA41 and TA51, similar to that shown in FIG. 10 titled“Distribution of SCK to an individual by an authentication centre” andits associated text in the Terrestrial Trunked Radio (TETRA); Voice plusData (V+D); Part 7: Security, EN 300 392-7 V2.1.1, 2000-12 (hereinreferred to as “TETRA Standard”), which is incorporated in its entiretyherein by reference. The KMF 101 stores an encryption process 301 thatcombines RSO and the appropriate KEK, KEKN, and KEK-VN utilizingencryption algorithms TA41 303 and TA51 305, yielding SKEK, which is asealed version of the KEK. RSO, SKEK, KEKN, and KEK-VN are forwarded tothe target system device. Curly brackets { } followed by a key nameindicate that the material within the curly brackets was created usingTA41 and TA51 and the key name after the brackets.

For example, KEK_(Z1) is intended to be transferred to the first zonecontroller 107 and BS1 115. RSO, KEK_(Z1), KEK_(Z1)-VN, and KEK_(Z1)N,and KEK_(ZC1) are combined utilizing encryption algorithms TA41 andTA51, yielding SKEK_(Z1). Key material RSO, SKEK_(Z1), KEK_(Z1)-VN, andKEK_(Z1)N are forwarded transparently through ZM1 105 to the first zonecontroller 107, which combines this key material with KI_(ZC1) usingTA41 and TA52 (as described in the TETRA Standard), yielding KEK_(Z1),which is stored at ZC1 107. RSO, KEK_(Z1), KEK_(Z1)-VN, and KEK_(Z1)N,and KI_(BS1) are combined utilizing encryption algorithms TA41 and TA51,yielding SKEK_(Z1). Key material RSO, SKEK_(Z1), KEK_(Z1)-VN, andKEK_(Z1)N are forwarded transparently through ZM1 105 to BS1 115, whichcombines this key material with KI_(BS1) using TA41 and TA52, yieldingKEK_(Z1), which is stored at BS 1115. In the preferred embodiment, anunencrypted acknowledgment of successful receipt of each key is returnedto the KMF 101 via the ATR 113.

A block diagram showing key storage within a communication system isshown in FIG. 4. In particular, storage of session authenticationinformation throughout the communication system is shown. In thepreferred embodiment, session authentication information includes arandom seed, RS, and two session keys, KS for authentication of an MSand KS′ for authentication of the infrastructure, for each mobilestation 401, 403, and 405 (only three are shown due to spaceconstraints, although numerous MSs are part of the system). The sessionauthentication information (SAI) is used to generate a derived cipherkey (DCK) for each MS 401.

For each MS 401, 403, and 405, the KMF 101 stores an Individual TETRASubscriber Identity (ITSI), TETRA Equipment Identity (TEI), and an MSauthentication key (“MS key”) that is unique to and stored within eachMS 401, 403, and 405. In the preferred embodiment, the air interfacekeys and the MS keys are stored in hardware encrypted fashion using ahardware key K_(H) within the KMF 101. The DVI-XL algorithm, availablefrom Motorola, Inc., is used to encrypt the keys for storage in the KMF101 in the preferred embodiment. Square brackets [ ] followed by a keyname indicate that the material within the square brackets is encryptedby that key.

The KMF 101 generates session authentication information for each MS401, 403, and 405, which SAI is at least partially encrypted andforwarded in non-real time to the UCS 103 for storage. For each MS 401,403, and 405, the UCS 103 stores the ITSI, TEI, and ID of the HLRassociated with each MS, as well as the SAI. In the preferredembodiment, KS and KS′ are stored encrypted by the interkey (as receivedfrom the KMF 101) at the UCS 103 for fast and easy transport, and RS isstored unencrypted. The UCS 103 is a transparent device in the preferredembodiment, thus it performs no encryption or decryption functions. Inorder to eliminate potential double entry of information, the KMF 101receives configuration information from the UCS 103. Examples ofconfiguration information are: Individual TETRA Subscriber Identity(ITSI), Group TETRA Subscriber Identity (GTSI), home zone, and zonemanagers. The KMF uses a table lookup, such as a DNS (Domain NameServer) lookup table, to obtain the ATR 113 and 127 addresses. Thedistribution of each of the different key types has differentconfiguration requirements, as described herein.

The UCS 103 forwards the appropriate SAI to each ZM 105 in non-realtime, based on the HLR ID associated with each MS 401. The ZM 105, likethe UCS 103, is a transparent device and performs no encryption ordecryption functions. The ZM 105 stores, for each MS having the HLR 109as its home location, an ITSI, TEI, and SAI. In the preferredembodiment, KS and KS′ are stored encrypted by the interkey (as receivedfrom the UCS 103) at the ZM 105 or 119 for fast and easy transport, andRS is stored unencrypted.

The ZM 105 forwards the SAI to the HLR 109 in non-real time. The HLR 109stores an ITSI and the SAI for each MS 401, 403, and 405. In thepreferred embodiment, KS and KS′ are stored encrypted by the interkey(as received from the ZM 103) at the HLR 109, and RS is storedunencrypted. In the preferred embodiment, RS, KS, and KS′ are storedunencrypted at the VLR 111 for faster authentication. In an alternativeembodiment, KS and KS′ may be stored unencrypted at the HLR 109 forfaster authentication.

When an MS 401 is authenticated at the zone, a new DCK for the MS 401 isgenerated by the VLR 111 at the zone controller 107 from the SAI in realtime, after any encrypted SAI is decrypted due to transfer of the SAIfrom the HLR 109. (The ITSI, SAI, and previous DCK associated with thatMS 401 are forwarded to the VLR 111 in real time before the new DCK iscreated.) The ITSI, SAI, and new DCK are forwarded to the HLR 109 inreal time for storage. In the preferred embodiment, the ITSI, SAI, andDCK comes from the HLR for the MS 401, thus this information may comefrom a different zone if the MS 401 does not use the HLR 109 for itshome. When the SAI/DCK comes from a different zone, that zonedecrypts/encrypts the information, as necessary, with the interkey fortransport to the appropriate zone, which also provides appropriatedecryption/encryption within the zone. DCK is stored encrypted by theintrakey KEK_(Z) for the zone in which it is stored, for easy and fasttransport to the local BS 115 or 117. In the example shown in FIG. 4,each DCK is stored encrypted by KEK_(Z1). In the preferred embodiment,KS and KS′ are always encrypted with the interkey KEK_(M), for fast andeasy transport during the authentication process, even when transfer iswithin the same zone.

During the authentication process, the BS 115 communicating with the MS401 receives, from ZC1 107 in real time, the MS's 401 DCK, encrypted bythe intrakey KEK_(Z1). The BS 115 stores the ITSI and DCK unencryptedfor immediate use while the MS 401 is at the coverage area of the BS115. See FIG. 17 and its associated text for information regarding keypersistence at each site.

Each MS 401, 403, and 405 stores its own ITSI, TEI, and DCK inunencrypted form, and K is stored in scrambled or encrypted form. EachMS 401, 403, and 405 also stores in unencrypted form relevant CCKs,GCKs, MGCKs, and SCKs as they are received. These keys may be storedencrypted in the infrastructure in an alternative embodiment.

The zone controller 107 is responsible for the real time distribution ofkeys and mobility management thereof. It maintains keys that may need tobe distributed in a real-time manner necessary when roaming, forexample. The group cipher key is an element in each talkgroup record andis kept in the talkgroup HLR. The common cipher key is a zone or sitespecific key and is maintained in the zone controller as well. The ZC isresponsible for the creation of the MGCK (based upon the GCK and CCK)and the distribution to the sites.

Because keys reside in the talkgroup and individual HLR 109, the zonecontroller 107 is not transparent with respect to the encryption of keymaterial. The ZC 107 maintains a protection key, KI, and twoinfrastructure key encryption keys, interkey KEK_(M) and intrakeyKEK_(Z), for the distribution of key material. KI is used to seal(encrypt) KEK_(M) and KEK_(Z) when they are sent from the KMF 101. Mostkey information is encrypted by the KMF 101 with the interkey, KEK_(M).The zone controller 107 decrypts the key material using KEK_(M) andre-encrypts the same information using KEK_(Z) when sending theinformation to a site within the zone. Thus, the zone controller 107 hasthe TETRA algorithms used for the encryption/decryption ofinfrastructure keys (such as TA41 and TA52 and TA31 and TA32), asdescribed herein.

The zone controller sends ACKs from infrastructure re-keying operationsto the KMF 101 via the ATR 113. When a ZC 107 or HLR 109 receives a keyupdate, the device first decrypts key update and checks for corruptionby verifying the integrity of the data and sends the result of thisoperation to the KMF 101 via the ATR 113 in the form of an ACK.

The site is one endpoint for air interface encryption. Audio on the airinterface between the BS 115 and MS 401 is encrypted. Audio within theinfrastructure is not encrypted. Outbound traffic is encrypted withalgorithms using MGCK, CCK, and SCK, or DCK for individual calls. Allinbound traffic is encrypted with algorithms using DCK or SCK. The sitemaintains the traffic algorithms and key storage for SCK, CCK, and MGCK,as well as DCK. Because the base site has traffic key storage, the basesite is not transparent with respect to the encryption of key material.All key material distributed to the base site is encrypted by theintrakey, KEK_(Z). Thus, the base site maintains a protection key, KI,and an interkey, KEK_(Z). Thus, the base sites have the TETRA algorithmsused for the encryption/decryption of infrastructure keys (such as TA41and TA52 and TA31 and TA32), as described herein. The MS is the otherendpoint point for air interface encryption. Outbound traffic isencrypted with algorithms using MGCK, CCK, and SCK, or the DCK ifindividually addressed. All inbound traffic is encrypted with algorithmsusing DCK or SCK, and identities may be encrypted with SCK or CCK. TheMS maintains the traffic algorithms and key storage for SCK, CCK, GCK,and MGCK as well DCK.

The following figures provides examples of the role of the zonecontroller 107 or 121 in some of its key generation, key distribution,and authentication functions, as well as the base site/base station andMS operations in the key generation, key distribution, andauthentication processes.

A diagram showing an example of key storage and authenticationinformation distribution within a communication system is shown in FIG.5. Session authentication information (RS, KS, and KS′) is needed tofacilitate real-time authentication of the MS 401 by the ZC 107 andreal-time authentication of the system by the MS, as well as mutualauthentication. Triggers for the transfer of SAI may be a manualinitiation by the KMF operator, an automatic fraud trigger from thesystem, or a periodic changing of the SAI by the KMF 101.

FIG. 5 shows the transfer of SAI for two mobile stations, ITSI1 401 andITSI2 403 (both not shown). The KMF 101 encrypts at least a part of theSAI (e.g., KS and KS′) with the interkey KEK_(M) for the system, andforwards ITS1, ITSI2, RS, and KS and KS′ encrypted by KEK_(M) to the UCS103. The UCS 103 stores a copy and forwards it to the home ZM 105 or 119for each ITSI. Dashed lines within a system device indicate transparentpassage of information through the system device. The ZM 105 or 119 alsostores a copy and forward it to its ZC 107 or 121, in particular, theHLR 107 or 123. The ZC 107 or 121 stores KS and KS′ encrypted along withRS in the HLR 107 or 123. Once the HLR 109 or 123 receives the SAI, anunencrypted acknowledgement (ACK) is sent, when decryption using KEK_(M)fails, back to the KMF 101 via the ATR 113 or 127 from the zone in whichthe HLR 109 or 123 resides. If a VLR 111 for the MS 403 exists, such asITSI2, the ZC 121 sends KS and KS′ encrypted with the interkey KEK_(M)to the VLR 111. Coordination between a previous authentication sessioninformation and a new authentication session information is not needed.The HLR 109 or 123 only needs one copy of SAI per ITSI registered. TheUCS 103 and ZM 105 or 119 store copies of authentication sessioninformation to provide recovery from system maintenance or failures.

By providing storage and forwarding of session authenticationinformation and keys in non-real time (i.e., without time constraint)between first-level system devices and in real time (i.e., on demand)between second-level system devices as described above, theauthentication system provides a fault tolerant system that allows forquick fault recovery as well. If the KMF 101, UCS 103, and/or ZMs 105and 119 fail or are separated from the rest of the system, fullauthentication may still be performed without interruption on areal-time basis with the session authentication information, for examplefor MS2 403, stored at the HLR 123 and VLR 111. A failure at any ofthese devices 101, 103, 105, and 119 is not catastrophic, in that thedata stored may be downloaded from any of the other devices that storesthe information. If a zone controller 107, HLR 109, and/or VLR 111experience a fault or failure, the SAI may be immediately downloadedfrom the ZM 105 at the zone. By eliminating the need for the KMF 101 toparticipate in real time in the authentication process, there is lessburden on the KMF 101 and less traffic in general on the communicationlinks between the system devices of the infrastructure.

A diagram showing authentication information storage and authenticationdecision making within a communication system is shown in FIG. 6. Fourmobile stations are shown within a system where three mobile stations401, 403, and 405 use HLR1 109 of the first zone controller 107, onemobile station 407 uses HLR2 123 of the second zone controller 121, twomobile stations 401 and 403 use VLR1 111, and two mobile stations 405and 407 use VLR2 125. Storage of SAI is shown throughout the systemdevices. Also shown are base station decisions whether or not toauthenticate a mobile at a particular trigger. For example, power-upmessages, whether encrypted or not, require authentication. Any messagesent in the clear (i.e., unencrypted) requires authentication. Encryptedroam messages may be implicitly authenticated, i.e., the challenge andresponse mechanism may be bypassed if the encrypted roam message issuccessfully decrypted by the BS 131. Power-up messages, roam messages,location updates, and other types of messages are considered requests tocommunicate within the communication system. When authentication isrequired, the BS 115, 117, 129, or 131 sends a request to authenticatethe MS to the infrastructure (to a zone controller in the preferredembodiment). In the event that the infrastructure device to whichauthentication requests are sent becomes unavailable, e.g., the devicefails, is down for maintenance, or the communication link to the deviceis not operable, the BS stores authentication requests during the timeperiod when the infrastructure device is not available. When theinfrastructure device becomes available, e.g., the device is returned toservice after a failure or maintenance or when the communication linkcomes up, the BS forwards the stored authentication requests to theinfrastructure device.

In one situation shown in FIG. 6, a first MS 401 sends a clear(unencrypted) power-up message to the first BS 115. In the preferredembodiment, authentication of the MS 401 in this situation is required.Because the MS 401 uses HLR 109 in the zone where the BS 115 is located,the session authentication information SAI1 for the MS 401 is forwardedfrom the HLR 109 to the VLR 111 at the zone for completion of theauthentication process.

The second MS 403 roams from BS1 115 to BS2 117 and sends a clear(unencrypted) roam message to the second BS 117. In the preferredembodiment, authentication of the MS 403 in this situation is required.Because the MS 403 uses the HLR 109 in the zone where the BS 115 islocated, and because the MS 403 roamed from a site serviced by the sameVLR as the new site, the session authentication information SAI2 for theMS 403 is already located in the VLR 111 at the zone for completion ofthe authentication process.

The third MS 405 sends an encrypted power-up message to the third BS129. In the preferred embodiment, authentication of the MS 405 in thissituation is required. Because the MS 405 uses the HLR 123 in the zonewhere the BS 129 is located, the session authentication information SAI3for the MS 405 is forwarded from the HLR 123 to the VLR 125 at the zonefor completion of the authentication process.

The fourth MS 407 roams from BS2 117 to BS4 131 and sends an encryptedroam message to the fourth BS 131. In the preferred embodiment, (full)authentication of the MS 403 in this situation is not required. Instead,the MS 407 is implicitly authenticated, i.e., the challenge and responsemechanism is bypassed if the encrypted roam message is successfullydecrypted by the BS 131. Because the MS 407 uses the HLR 109 in the zoneother than the zone where the BS 131 is located, the encryption key (andif necessary, the session authentication information SAI4) for the MS407 must be forwarded from that HLR 109 to the VLR 125 where the MS 407has roamed for completion of the authentication process. Typically, atleast a part of the SAI is encrypted by the interkey prior to transferto another zone. If implicit authentication fails, full authenticationof the MS 407 is then performed.

A diagram showing the challenge-and-response process to authenticate amobile station by an authentication center in accordance with the TETRAStandard is shown in FIG. 7. When authenticating an MS 707, anauthentication center 701, such as a KMF 101, combines the mobileauthentication key, K, with RS utilizing the encryption algorithm TA11,as defined in the TETRA Standard. The output of the TA11 process 703 isKS, which is input with RAND1 (a random number) to the encryptionalgorithm TA12, as defined in the TETRA Standard. The TA12 process 705outputs XRES1, an expected response, and DCK1, a derived cipher key forthe mobile. RAND1 and RS are provided to the MS 707. The MS 707 goesthrough a similar process, by combining its mobile authentication key,K, with RS received from the AuC 701 utilizing the TA11 process 703. TheTA11 process 703 outputs KS, which is input with RAND1 to the TA12process 705. The TA12 process 705 in the MS 707 outputs RES1, a responseto the challenge, and DCK1, the derived cipher key for the mobile. TheMS 707 forwards RES1 to the AuC 701. If XRES1 and RES1 match, the AuC701 sends an authentication pass message to the MS 707, andcommunication over the air interface with the newly created DCK1 maycommence. If XRES and RES do not match, the AuC 701 sends anauthentication fail message to the MS 707, and communication over theair interface with the newly created DCK1 is prohibited, although theold DCK1 may be used upon authentication failure.

A diagram showing the challenge-and-response process to authenticate anauthentication center by a mobile station in accordance with the TETRAStandard is shown in FIG. 8. When authenticating an AuC 701, such as aKMF 101, an MS 707 combines the mobile authentication key, K, with RSutilizing the encryption algorithm TA21, as defined in the TETRAStandard. The TA21 process 801 outputs KS′, which is input with RAND2 (arandom number) to the encryption algorithm TA22, as defined in the TETRAStandard. The TA22 process 803 outputs XRES2, an expected response, andDCK2, a derived cipher key for the mobile 707. RAND2 is provided to theAuC 701. The AuC 701 goes through a similar process, by combining themobile authentication key, K, for the MS 707 with RS utilizing the TA21process 801. The TA21 process 801 of the AuC 701 outputs KS′, which isinput with RAND2 to the TA22 process 803. The output of the TA22 process803 in the AuC 701 is RES2, a response to the challenge, and DCK1, thederived cipher key for the mobile. The AuC 701 forwards RES and RS tothe MS 707. If XRES and RES match, the MS 707 sends an authenticationpass message to the AuC 701, and communication over the air interfacewith the newly created DCK1 may commence. If XRES and RES do not match,the MS 707 sends an authentication fail message to the AuC 701, andcommunication over the air interface with the newly created DCK1 doesnot take place.

A diagram showing SAI distribution and the authentication processbetween a communication system and a mobile station in real time inaccordance with the invention is shown in FIG. 9. FIG. 9 shows animplementation of the authentication process of the TETRA Standardincluding how various system devices within the infrastructure performwithin the authentication process. FIG. 9 shows how the ZC 107,including the HLR 109 and VLR 111, and BS 115 act as proxies, orauthentication agents, for the KMF 101 in the authentication process. Innon-real time, KS and KS′ encrypted by the interkey, and RS are passedalong from the KMF 101 to the UCS 103, to the first ZM 105, and to theHLR 109 of the first zone controller 107.

After the BS 115 sends a request for authentication of the MS 401 to theZC 107, the VLR 111 generates RAND1 and uses KS and RAND1 with the TA12process to generate XRES1 and DCK1, in accord with FIG. 7 herein, andforwards RAND1 and RS to the BS 115, which forwards RAND1 and RS overthe air to the MS 401. The MS 401 combines its own K and RS with theTA11 process to generate KS, then combines RAND1 and KS in accord withFIG. 7 herein, yielding RES1 and DCK1, and forwards RES1 to the BS 115,which forwards RES1 to the VLR 111 at the ZC 107. The VLR 111 comparesRES1 and XRES1, and the result is R1. When RES1 and XRES1 match, DCK1and the SAI for the MS 401 are stored in the VLR 111 and HLR 109 andDCK1 (encrypted by the interkey). In the preferred embodiment, DCK1 isencrypted with the intrakey for the first zone prior to being sent tothe BS 115. R1 is forwarded to the BS 115 in acknowledgment thatauthentication passed, and the BS 115 stores DCK1 and sends R1 to the MS401 indicating authentication has passed. When RES1 and XRES1 do notmatch, the VLR 111 discards the newly created DCK1 without storing orforwarding to the BS 115 and forwards R1, a negative acknowledgment ofthe authentication process, to the BS 115, and the BS 115 sends R1 tothe MS 401 indicating authentication has failed.

To request authentication of the infrastructure, the MS 403 sends RAND2to the BS 129, which forwards RAND2 to the VLR 125 in the ZC 121. TheVLR 125 looks up RS and KS′ and generates RES2 and DCK2 using the TA22process in accord with FIG. 8 herein, and forwards RES2 and RS to the BS129, which forwards RES2 and RS over the air to the MS 403. The MS 403combines RS and its own K with process TA21, yielding KS′, which is thencombined with RAND2 in the TA22 process in accord with FIG. 8 herein,yielding XRES2 and DCK2. The MS 403 compares RES2 and XRES2. When RES2and XRES2 match, the MS 403 sends message R2 to the BS 129 inacknowledgment that authentication passed, the BS 129 sends R2 to the ZC121, and the VLR 125 causes DCK2 and the SAI for the mobile 403 to bestored in the VLR 125 and the HLR 123 for the MS 403 and forwards DCK2to the BS 129, which stores DCK2. In the preferred embodiment, DCK2 isencrypted with the intrakey for the second zone prior to being sent tothe BS 129. When RES2 and XRES2 do not match, the MS 403 sends messageR2 to the BS 129 indicating that authentication failed, the BS 129 sendsR2 to the ZC 121, and the VLR 125 discards the newly created DCK2without sending it to the BS 129.

In either authentication process, if the VLR 111 in the zone where theMS 401 or 403 is presently located does not have SAI stored for the MS401 or 403, the VLR 111 obtains the SAI from the HLR for the MS 401 or403. When the HLR 109 for the MS 401 or 403 is in the same zone, the SAIis simply passed within the ZC 107 to the VLR 111. When the HLR 109 forthe MS 401 or 403 is in a different zone, the zone for the home HLR isdetermined from a home zone mapping table that maps ITSI to its HomeZone, and the SAI is forwarded to the ZC 107 to the VLR 111. In thepreferred embodiment, when the key material is forwarded from the HLRfor the MS 401 or 403 to the VLR 111, at least some of the SAI, inparticular KS and KS′, are encrypted with the interkey. When DCK istransferred within a zone, DCK is encrypted with KEK_(Z). Similarly, ifthe zone where authentication takes place is not the home zone for theMS 401 or 403, updated SAI and DCK information will be inter keyencrypted, at least in part, and forwarded to the appropriate VLR. Askeys are passed between devices that require a different encryption key,one device receives a message, decrypts it with one key, and re-encryptsthe result with another key for the next device. Mutual authentication,when the MS and infrastructure mutually authenticate each other, isdescribed with respect to FIG. 3 titled “Mutual authentication initiatedby SwMI” and FIG. 4 titled “Mutual authentication initiated by MS” andtheir associated text of the TETRA Standard. The resultant DCKs (DCK1and DCK2) of each process are combined using the TB4 encryptionalgorithm, and the resulting DCK is used to communicate.

A diagram showing a key pull within a communication system is shown inFIG. 10. The key pull procedure is used to forward an air interface key,typically the DCK, although the process may also be used for GCK/MGCK,into a BS that does not have the DCK for a mobile station. Thissituation may occur when an MS switches sites while idle or a failurearises. FIG. 10 shows MS1 401 switching from site 1 to site 2 withinzone 1 and MS2 403 roaming from zone 2 to zone 1. Although KS, KS′, andDCK are stored encrypted at the HLR, and DCK is stored encrypted at theHLR and VLR in the preferred embodiment, they are shown unencrypted inFIG. 10 for the sake of simplicity.

MS1 401 has roamed from site 1 to site 2 in zone 1. The pull procedureis initiated by the BS 117 when it recognizes that it does not have theDCK for the MS 401 that has sent an encrypted message, for example, aDCK-encrypted location update message. The BS 117 may optionally forwardan acknowledgment of receipt of the encrypted message to the mobilestation 401. The identity, ITSI1, of the MS 401 is encrypted with CCK,so the BS 117 is able to determine which MS has sent the message, eventhough it does not have DCK1 for the MS 401. The BS 117 requests theDCK1 from the ZC 107. The ZC 107 determines if it needs to request DCK1from a different zone. In this case, because MS1 401 is roaming withinthe same zone, DCK1 is found in the VLR 111, and the ZC 107 sends DCK1to the BS 117 encrypted with the intrakey KEK_(Z1). The BS 117 uses DCK1to decrypt the location update message for MS1 401, and any subsequentmessage(s) from the MS 401, and forwards the location update to the ZC107. In the preferred embodiment, the VLR 111 for the MS 401 is notupdated with the MS location until the MS implicitly authenticates orperforms a full authentication. Receipt of a properly decrypted locationupdate message is considered an implicit authentication, at which timethe VLR 111 would be updated.

MS2 403 has roamed from zone 2 to zone 1. The pull procedure isinitiated by the BS 115 when it recognizes that it does not have the DCKfor the MS 403 that has sent an encrypted message, for example, aDCK-encrypted location update message. The BS 115 may optionally forwardan acknowledgment of receipt of the encrypted message to the mobilestation 403. The identity, ITSI2, of the MS 403 is encrypted with CCK,so the BS 115 is able to determine which MS has sent the message, eventhough it does not have DCK2 for the MS 403. The BS 115 requests theDCK2 from the ZC 107. The ZC 107 determines if it needs to request DCK2from a different zone, which is required in this case, because MS2 403is roaming from a different zone, zone 2, and the HLR 123 for the MS 403is in zone 2. The ZC 107 determines which zone has the needed keymaterial and sends a request to that target zone for the key material.In the example, DCK2 is found in the HLR 123 for zone 2, which is thetarget zone, and DCK2 is sent to the ZC 107 from that zone's HLR 123after being encrypted with interkey, KEK_(M). The ZC 107 sends DCK2 tothe BS 115 encrypted with the intrakey KEK_(Z1). The BS 115 uses DCK2 todecrypt the location update message for MS2 403, and any subsequentmessage(s) from the MS 403, and forwards the location update to the ZC107. RS, KS, KS′ are requested at a later time from the HLR 123 so thata full authentication may be performed as necessary. In the preferredembodiment, the VLR 111 for the MS 403 is not updated with the MSlocation until the MS implicitly authenticates or performs a fullauthentication. Receipt of a properly decrypted location update messageis considered an implicit authentication, at which time the VLR 111would be updated.

In the situation where it may be desired to pull a GCK/MGCK, the processis the same as described above with respect to the DCK, except that theVLR 111 obtains the GCK, combines it with a CCK, as described below inFIG. 15 and its associated text, and forwards the resultant MGCK,encrypted with the intrakey KEK_(Z1),to the BS 115 or 117.

A diagram illustrating a key push within a communication system is shownin FIG. 11. The key push procedure is used to forward a key, such as theDCK or GCK/MGCK, to a forwarding site when an MS switches sites from itscurrent site to the forwarding site. This process thus provides amechanism for a key to be forwarded to a site prior to the arrival ofthe MS 401 or 403, so that seamless encrypted handoffs and roaming mayoccur. FIG. 11 shows an example of a transfer of DCK2 between zones anda transfer of DCK1 within a zone. The MS initiates the procedure.Although KS, KS′, and DCK are stored encrypted at the HLR, and DCK isstored encrypted at the HLR and VLR in the preferred embodiment, theyare shown unencrypted in FIG. 11 for the sake of simplicity. MS1 401begins the process of roaming from BS1 115, having Location AreaIdentification 1 (LAID1), at site 1 to BS2 117, having Location AreaIdentification 2 (LAID2) at site 2 at zone 1. The MS 401 sends to BS1115 a message indicating that MS 1 will roam to site 2. In the preferredembodiment, this message is an OTAR Prepare message. The BS 115 relaysthis message to the ZC 107. The ZC 107 determines if the DCK needs to betransferred to another zone or not by determining whether or not thesite to which the MS 401 is roaming is in its zone or not. In thisexample, site 2 is also serviced by the ZC 107, thus there is no need totransfer the DCK to another zone. Because the DCK is transferred withinthe zone, the ZC 107 responds to the BS 115 with a use short delaymessage. In this case, the BS 115 holds off the MS 401 from switching tosite 2 by a delay equivalent to the short delay, which delayapproximates the time it will take to forward DCK to the next site fromthe VLR 111 in the same zone. In the preferred embodiment, the shortdelay is less than 50 ms. The MS 401 waits for an ok from the BS 115before operating at the new site, e.g., roaming, switching sites, orcommunicating, and the BS 115 sends the ok after the short delay periodexpires. During the delay period, the VLR 111 at ZC1 107 encrypts DCK1with the intrakey and forwards it to BS2 117 at site 2, where the MS 401and BS2 117 will be able to exchange encrypted messages using DCK1. Inthe preferred embodiment, the VLR 111 for the MS 401 is not updated withthe MS location until the MS 401 implicitly authenticates or performs afull authentication.

MS2 403 begins the process of roaming from BS3 129, having Location AreaIdentification 3 (LAID3) at site 3 at zone 2 to BS1 115, having LocationArea Identification 1 (LAID1) at site 1 at zone 1. The MS 403 sends toBS3 129 a message indicating that MS2 will roam to site 1. In thepreferred embodiment, this message is an OTAR Prepare message. The BS129 relays this message to the ZC 121. The ZC 121 determines if the DCKneeds to be transferred to another zone or not by determining whether ornot the site to which the MS 401 is roaming is in its zone or not. Inthis example, site 1 is not serviced by the ZC 121, thus there is a needto transfer the DCK to another zone. Because the DCK is transferred toanother zone, the ZC 121 responds to the BS 129 with a use long delaymessage. In this case, the BS 129 holds off the MS 403 from switching tosite 1 by a delay equivalent to the long delay, which delay approximatesthe time it will take to forward DCK from the VLR 111 to the site in thenext zone. In the preferred embodiment, the long delay is greater thanor equal to 50 ms. The MS 403 waits for an ok from the BS 129 beforeswitching sites, and the BS 129 sends the ok after the long delay periodexpires. During the delay period, the VLR 125 at ZC1 121 encrypts DCK2with the interkey and forwards it to ZC1 107, which decrypts it with theinterkey, encrypts it with the intrakey KEK_(Z1), and forwards theresult to BS1 115 at site 1, where the MS 403 and BS2 115 will be ableto exchange encrypted messages using DCK2. In the preferred embodiment,the VLR 111 for the MS 403 is not updated with the MS location until theMS 403 implicitly authenticates or performs a full authentication, atwhich time the VLR 125 for MS2 in ZC2 121 is eliminated. RS, KS, KS′ arerequested at a later time from the HLR at ZC3 223 (the home zone HLR forthe MS 403) so that a full authentication may be performed as necessary.

FIG. 12 is a diagram showing distribution of a static cipher key to a BSwithin a communication system. The SCK is a system wide voice traffickey that is used to encrypt voice, data, ESI (encrypted short identity),and signaling traffic when authentication is not available. SCKs areidentified by SCKN and SCK-VN, and are stored in the KMF 101 encryptedby a hardware key and in the ZMs 105 and 119 encrypted by TA31. In thepreferred embodiment, there may be up to 32 distinct SCKs in the entiresystem. Each BS stores one SCK, identified by SCK number (SCKN), each ofwhich has an SCK version number (SCK-VN), although SCK may have multipleversions that are or were used in the system. Each SCKN has a versionnumber SCK-VN, and in the preferred embodiment, two version numbers,i.e., two keys, are stored for each SCKN. The MS must be able to store32 SCKs for one SCK-VN, in addition to 32 SCKs for another SCK-VN. The31 additional SCKs in the MS are defined for direct operation betweenmobile stations. A new SCK replaces the oldest SCK-VN. The SCK may beprovided to BSs and mobile stations in several ways, including via a KeyVariable Loader (KVL), via computer software such as RSS Softwareavailable from Motorola, Inc., and via OTAR (Over-the-Air Rekeying) viathe home zone ATR of the MS. Although not shown in the drawing becauseof space constraints, SCKN and SCK-VN are sent along with SCK foridentification purposes.

A process to transfer an SCK to each BS in the system is shown in FIG.12. When the KMF 101 determines that an SCK update is due, the KMF 101generates a new SCK. In order to determine the home zone of a BS, in thepreferred embodiment, the KMF 101 uses the BS to home ZC map from theUCS 103 and a table lookup based on the zone to obtain the address forthe ATR in the zone. The KMF 101 encrypts the SCK with the intrakey,KEK_(Z), for the zone in which the BS is located, and sends theencrypted key to the ZM for that BS. The ZM stores a copy and forwardsit to the intended BS. An unencrypted ACK is sent from the BS to the ZCand to the KMF 101 via the ATR in the zone where the BS resides. The ACKrepresents that the SCK was received correctly in the BS.

A specific example of an SCK transfer to BS1 115 includes a transfer ofsite information, including an BS to home zone controller map, from theUCS 103 to the KMF 101. The KMF 101 uses the map to determine that BS1115 is located in zone 1. The KMF 101 generates the SCK and encrypts itwith the intrakey, KEK_(Z1), for zone 1 where BS1 is located. The KMF101 forwards the encrypted SCK to the ZM 105 for zone 1. ZM1 105 storesa copy of the encrypted SCK and forwards it to BS1 115 via a wirelinelink. BS1 115 decrypts the encrypted SCK using KEK_(Z1) and stores theSCK unencrypted. When the SCK is received correctly by BS1, BS1 115sends an unencrypted ACK to the KMF 101 via ZC1 107 and the ATR 113 inzone 1. Transfers of SCK to BS3 and BS4 are similarly performed.

A diagram showing distribution of a static cipher key to a mobilestation within a communication system is shown in FIG. 13. When the KMF101 determines that an SCK update for an MS 401 is due, the KMF 101generates a new SCK key material for the MS 401 according to FIG. 10titled “Distribution of SCK to an individual by an authenticationcenter” and its associated text in the TETRA Standard. The SCKgeneration process yields the key material SSCK (a sealed SCK), SCKN(SCK number), SCK-VN (SCK version number), and RSO (the random seed usedin the process). In order to determine the ATR for the home zone of theMS 401, in the preferred embodiment, the KMF 101 uses the ITSI to homeZC map from the UCS 103 and a table lookup based on the zone to obtainthe address of the ATR for the home zone. In the example of FIG. 13, thehome zone for MS1 401 is zone 2. The KMF 101 forwards SSCK, SCKN,SCK-VN, and RSO to the ATR 127 of the home zone (2) for the MS 401. Ifthe MS 401 is not on the system, the ATR 127 sends a NACK back to theKMF 101. If the MS 401 is on the system, the SCK is delivered to the MS401 via the zone in which the MS 401 is currently located. In thepreferred embodiment, the SCK key material (e.g., SSCK, SCKN, SCK-VN,and RSO) are not encrypted for transfer among system devices. The SCKkey material may optionally be encrypted for transfer among systemdevices.

When the MS 401 is not located in its home zone, the home zonecontroller 121 of zone 2 determines which zone the MS 401 is currentlylocated in (zone 1 in FIG. 12) by looking it up in the HLR 123 of zone2. ZC2 121 forwards SSCK, SCKN, SCK-VN, and RSO to the zone controller107 of the zone where the MS 401 is presently located. ZC1 107 forwardsSSCK, SCKN, SCK-VN, and RSO to the BS 115 where the MS 401 is located.The BS 115 decrypts the SSCK, SCK-VN, and RSO with the intrakey,KEK_(Z1), and forwards the result to the MS 401. An unencrypted ACK issent from the MS 401 to the BS 115 to the ZC 107 and to the KMF 101 viathe ATR 113 in the zone where the BS 115 resides. The ACK representsthat the SCK was received and unsealed correctly in the MS (theunsealing process is described in the TETRA Standard).

When the MS 401 is located in its home zone (not shown, but assumed tobe at BS3 129 for the sake of this example), the VLR of the home zonecontroller 121 forwards SSCK, SCKN, SCK-VN, and RSO to the BS 129 wherethe MS 401 is located (not shown but assumed for this example). The BS129 forwards SSCK, SCKN, SCK-VN, and RSO to the MS 401. An unencryptedACK is sent from the MS 401 to the BS 129 to the ZC 121 and to the KMF101 via the ATR 127 in the zone where the BS 115 resides. The ACKrepresents that the SCK was received and unsealed correctly in the MS(the unsealing process is described in the TETRA Standard).

FIG. 14 is a diagram showing distribution of a common cipher key to amobile station and a BS within a communication system. The CCK is alocation area based traffic key that is used to encrypt voice, data, andsignaling within a location area (LA) and is only used for outboundcommunications. The CCK is meant for use with the encryption of groupcall traffic in the TETRA Standard. The CCK is also used to encrypt thesubscriber identity creating the encrypted short identity (ESI). Groupcall traffic within the LA uses the CCK when there is no GCK availableor it is disabled. There is one CCK per location area. A location areamay be a small as a site, thus there could be as many as CCKs as sitesin the system. It is possible for more than one location area to havethe same CCK. CCK is identified by CCK-ID (e.g., CCK1, CCK2, and soforth) and LAID (location area identification). Two copies of each CCK(the latest two CCK-IDs) are in the ZC and the BS to enable a gradualrekeying of the MS in the system. While one CCK is in use, the next oneis distributed to the MS. In the preferred embodiment, each sitemaintains a CCK for each site adjacent to the site for seamless handoffsbetween sites and to facilitate consistent mobility management. When anadjacent CCK is given to an MS, the latest two CCKs are transferred tothe MS. A new CCK replaces the oldest CCK-ID. Long term storage of CCKsoccurs in the ZMs 105 and 119. The TETRA Standard supports severalmethods to provision CCK over-the-air, and the same request/providemethodology used for each of the air interface keys, and also allows keyrequest upon registration and cell change by the mobile station.

The CCK to BS procedure illustrated in FIG. 14 is used to transfer a CCKfrom the KMF 101 to a BS (site) 115. The KMF 101 determines that it istime for the CCK of a BS 115 to be updated and generates appropriateCCK(s). In the preferred embodiment, each BS is a Location Area (LA) andhas its own Location Area Identification (LAID). FIG. 14 shows thetransfer of CCK1 and CCK2 to zone 1 and the transfer of CCK3 to zone 2.The CCKs are encrypted with the intrakey, KEK_(Z), for the zone wherethe LA is located. The UCS 103 provides a site-to-zone map and anZM-to-zone map to the KMF 101. The KMF 101 uses these maps to send thekeys directly to the appropriate ZM 105 or 119, which stores CCK andforwards CCK to the zone controller 107 or 121. The UCS 103 obtains thesite parameters from the ZMs 105 and 119 to create the adjacent sitelist that is sent to the KMF 101 and forwarded the ZMs 105 and 119 to beforwarded to the zone controllers 107 and 121 for use. If an adjacentsite is in a different zone, the key is transferred between the involvedZCs. The ZC encrypts the CCK with the interkey, KEK_(M), for transferbetween zone controllers. Using the adjacent site list, the zonecontrollers 107 and 121 send the adjacent site CCKs to the appropriatesites. Thus, each site on the adjacent site list will have the CCKs forsites adjacent to that site. The adjacent CCKs are used so that the MSmay request the CCK for the adjacent site before the MS switches sites.The BS 115 may also forward CCKs to MSs as new CCKs are received at theBS 115. CCKs are encrypted with DCK for the particular MS 401 prior totransmitting the encrypted CCK to the MS 401. ACKs are sent by the BS toZC and are returned to the KMF 101 via the ATR (where the BS resides).Because the KMF 101 is unaware of adjacency, it does not need ACKs fromadjacent distributions of CCK. Because the KMF 101 tracks which BS isgiven a CCK, the BS tracks the currency of the CCKs, i.e., which MS hasa CCK for a given Location Area, and forwards ACKs once the CCK iscurrent.

Because MGCK is a combination of CCK and GCK, the zone controller willcreate four MGCKs using the latest two CCK-IDs and the latest twoGCK-VNs and distribute them accordingly (see FIG. 15 and FIG. 16).

The CCK is a zone specific parameter so there is no need to go throughthe UCS 103. Thus, the KMF 101 sends the CCK information directly to theappropriate zone manager 105 or 119, which is different than there-keying methodology of other air interface keys. The UCS 103 obtainsthe site information from the zone managers 105 or 119 to create theadjacent site list. By placing CCKs at adjacent sites, real-timeprocessing of CCKs is reduced, i.e., the BS does not need to query thezone controller for the CCK for an adjacent BS when an MS requests a CCKfor a neighboring site, thus the MS need not process a CCK when the MSswitches sites.

FIG. 15 is a diagram showing distribution of a group cipher key to a BSwithin a communication system. GCK is identified by GTSI (Group TETRASubscriber ID as referred to in the TETRA standard) and GCK-VN. In thepreferred embodiment, GCKN is logically equivalent to GTSI from a keymanagement perspective. Long term storage of GCK occurs in the UCS andZM. MGCK, which is a combination of GCK and CCK, is identified by GTSI(or GCKN), CCK-ID (with LAID), and GCK-VN. Four MGCKs per talkgroup(GTSI) are identified by the latest two CCK-Ids and the latest twoGCK-VNs. MGCKs are not stored in a ZC 107 or 121, but are created by aZC 107 or 121 and sent to the BS 115 provided that an MS affiliated withthat GTSI is at the site of the BS 115, which does not receive the GCKbecause it is a long-term key. Although not shown in the drawing becauseof space constraints, GCK-VN is sent along with GCK and MGCK foridentification purposes.

The procedure to update a GCK for a talkgroup record has two parts. Thefirst part includes updating the actual GCK in the for the talkgroup,the second part includes generating the resultant MGCK as a result ofthe update and distributing the MGCK to the sites.

The procedure of FIG. 15 transfers a GCK from the KMF 101 to thetalkgroup HLR in the zone controller at the home zone for the talkgroup.When the KMF 101 determines that it is time for the GCK to be updated,the KMF 101 generates a GCK for each talkgroup and maintains a GTSI-GCKtable. The GCKs are stored hardware encrypted at the KMF 101. The KMF101 does not know which ZC has the HLR for the GTSI, so the KMF 101sends the GCK encrypted with the interkey, KEK_(M), to the UCS 103. TheUCS 103 stores the key material and forwards it to the home ZM 105 or119 for the talkgroup (GTSI) associated with the GCK. The ZM 105 or 119forwards the key material to its ZC 107 or 121, which stores the keymaterial in the group HLR for GTSI encrypted by KEK_(M). The ZC 107verifies that the key material can be decrypted correctly and sends anACK back to the KMF 101 via the ATR 113 where the group HLR 109 for GTSIresides. The ACK reflects that the HLR 109 contains a correct encryptedcopy of the GCK. The ZC 107 decrypts the key material with KEK_(M) andre-encrypts it with the intrakey, KEK_(Z), for storage in the VLR 111.Any other VLRs, such as VLR2 125, outside of the home zone associatedwith the GTSI will have GCK encrypted with KEK_(M) forwarded to them.FIG. 15 shows both the inter-zone and intra-zone cases.

Because MGCK is a combination of GCK and CCK generated by a ZC using theTA71 algorithm 1501, 1503, or 1505, when GCK changes or CCK changes, theMGCK must also change accordingly. The four MGCKs are sent to all siteshaving a talkgroup affiliation matching the GTSI for GCK. Because thelatest 2 CCK-IDs and latest 2 GCK-VNs are stored, four versions of theMGCK need to be sent to the BS.

As in other cases, when sending MGCK to a site, it needs to be encryptedusing the intrakey, KEK_(Z) The GCK is obtained from the VLR talkgrouprecord and decrypted with the intrakey, KEK_(Z), and combined with CCKto create MGCK. The resultant MGCK is encrypted using the intrakey,KEK_(Z), and sent to the appropriate sites.

Transfer of an MGCK to a BS may be triggered by a number of events.Examples of triggers include a mobile station associated with the GCKfor the MGCK residing at the BS when the either the GCK or CCK isgenerated; a mobile station arriving at the BS when no previoustalkgroup affiliation at that BS had occurred; and a mobile stationchanging talkgroup affiliation, while residing at the BS, to a talkgroupnot previously associated with the BS.

A diagram showing distribution of a group cipher key to a mobile stationwithin a communication system is shown in FIG. 16. When the KMF 101determines that an GCK update for an MS 401 is due, the KMF 101generates a new GCK key material for the MS 401 according to FIG. 8titled “Distribution of a group cipher key to an individual” and itsassociated text in the TETRA Standard. The GCK generation process yieldsthe key material SGCK (a sealed GCK), GCKN (GCK Number), GCK-VN (GCKversion number), and RSO (the random seed used in the process). In orderto determine the ATR for the home zone of the MS 401, in the preferredembodiment, the KMF 101 uses the ITSI to home ZC map from the UCS 103and a table lookup based on the zone to obtain the address of the ATRfor the home zone. In the example of FIG. 16, the home zone for MS1 401is zone 2. The KMF 101 forwards SGCK, GCKN, GCK-VN, and RSO to the ATR127 of the home zone (2) for the MS 401. If the MS 401 is not on thesystem, the ATR 127 sends a NACK back to the KMF 101. If the MS 401 ison the system, the GCK is delivered to the MS 401 via the zone in whichthe MS 401 is currently located. In the preferred embodiment, the GCKkey material (e.g., SGCK, GCKN, GCK-VN, and RSO) are not encrypted fortransfer among system devices. The GCK key material may optionally beencrypted for transfer among system devices.

When the MS 401 is not located in its home zone, the home zonecontroller 121 of zone 2 determines which zone the MS 401 is currentlylocated in (zone 1 in FIG. 16) by looking it up in the HLR 123 of zone2. ZC2 121 forwards SGCK, GCKN, GCK-VN, and RSO to the zone controller107 of the zone where the MS 401 is presently located. ZC1 107 forwardsSGCK, GCKN, GCK-VN, and RSO to the BS 115 where the MS 401 is located.The BS 115 forwards SGCK, GCKN, GCK-VN, and RSO the MS 401. Anunencrypted ACK is sent from the MS 401 to the BS 115 to the ZC 107 andto the KMF 101 via the ATR 113 in the zone where the BS 115 resides. TheACK represents that the GCK was received and unsealed correctly in theMS (the unsealing process is described in the TETRA Standard).

When the MS 401 is located in its home zone (not shown, but assumed tobe at BS3 129 for the sake of this example), the home zone controller121 forwards SGCK, GCKN, GCK-VN, and RSO to the BS 129 where the MS 401is located (not shown but assumed for this example). The BS 129 forwardsSGCK, GCKN, GCK-VN, and RSO to the MS 401. An unencrypted ACK is sentfrom the MS 401 to the BS 129 to the ZC 121 and to the KMF 101 via theATR 127 in the zone where the BS 115 resides. The ACK represents thatthe GCK was received and unsealed correctly in the MS (the unsealingprocess is described in the TETRA Standard).

FIG. 17 is a flowchart showing a method of key persistence at a site ina communication system in accordance with the invention. Key persistencerefers to the time a key remains stored at any system device or MS. Ifan air interface traffic key is deleted from a site when the MS leavesthe site, and the key is removed too quickly, the MS may return to thesite requiring the key to be set up again. If the MS is travelingbetween zone borders or site boundaries for a period of time, the keymaterial for the MS may need to be constantly set up if the key materialis deleted from a site too quickly after the MS leaves the site. If thekey material is left at a site for too long, duplicate keys may be setup, creating ambiguity and the likelihood of authentication failures,particularly for implicit authentication. Thus, the key persistence foreach key needs to be set adequately to prevent such problems. In thepreferred embodiment, the persistence time is based on an expectedaverage authentication rate in the communication system, and preferablythe persistence time is less than the expected average authenticationrate in the communication system. The expected average authenticationrate is based on an average number of times a mobile stationauthenticates within a time period.

At step 1701, when a MS arrives at a site, key(s) and/or key materialassociated with the MS 401 are stored at the site. If at step 1703 it isdetermined that the mobile has left the site, a persistence timer is setat step 1705, unless it had already been set or reset, in which case theprocess simply continues with step 1709. When the timer expires at step1707, the process continues with step 1709 where the key(s) and/or keymaterial associated with the mobile 401 are deleted from the site, andthe process ends. If the mobile 401 has not left the site at step 1703,and it is time to replace the mobile's key(s) and/or key material atstep 1711, the key(s) and/or key material are replaced at step 1713 andthe process continues with step 1703. Step 1709 may also be reached (notshown) if a system device, such as a zone controller, directs the siteto delete certain key(s) and/or key material for any reason. The zonecontroller typically determines when the mobile leaves a site based onHLR and VLR updates.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges that come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A method comprising the steps of: dividing a plurality ofinfrastructure system devices other than a mobile station into aplurality of pools; utilizing an intrakey to encrypt messages passedbetween infrastructure system devices in the same pool; and utilizing aninterkey to encrypt messages passed between infrastructure systemdevices of different pools.
 2. The method of claim 1, wherein each ofthe plurality of pools comprises a mutually exclusive subset of theplurality of infrastructure system devices.
 3. The method of claim 1,wherein the messages comprise at least one encryption key.
 4. The methodof claim 1, wherein the messages comprise session authenticationinformation.
 5. The method of claim 1, wherein each different poolutilizes a different intrakey.
 6. The method of claim 1, wherein onlyone infrastructure system device from each pool utilizes the interkey.7. The method of claim 1, wherein the plurality of infrastructure systemdevices are part of a communication system infrastructure that providesencrypted communications.
 8. The method of claim 1, wherein at least oneof the plurality of infrastructure system devices has its own protectionkey, which protection key is utilized to encrypt and decrypt any of theintrakey and the interkey for transport to any of the otherinfrastructure system devices.
 9. The method of claim 1, wherein eachpool of the plurality of pools is comprised of one or moreinfrastructure system devices that reside in a single zone of aplurality of zones in a communication system.
 10. The method of claim 9,wherein the one or more infrastructure system devices that reside in asingle zone are comprised of at least one of a base station, a basesite, a TETRA site controller, and a zone controller.
 11. The method ofclaim 9, wherein only a zone controller within each of the plurality ofzones stores the interkey.
 12. The method of claim 1, wherein theinterkey is utilized to encrypt messages passed between aninfrastructure system device and a key management facility.
 13. Themethod of claim 1, wherein a message is encrypted by one of the intrakeyand the interkey based on an infrastructure system device to which themessage is forwarded.
 14. The method of claim 1 further comprising thesteps of: storing a protection key for each of the plurality ofinfrastructure system devices; and when transporting key material to aninfrastructure system device of the plurality of infrastructure systemdevices, encrypting the key material with the protection key associatedwith the infrastructure system device.
 15. The method of claim 14,wherein the key material is a key encryption key.
 16. The method ofclaim 14, wherein each of the plurality of infrastructure system deviceshas its own unique protection key.